Weighing a number of objects in succession

ABSTRACT

To weigh objects in quick succession, the objects are projected with a known horizontal velocity onto a ramp which has a surface which curves down through 90°. The ramp can move horizontally in response to the impulse corresponding to the change in horizontal momentum of the object. A transducer senses the movement of the ramp and the output horizontal velocity of the object is sensed, enabling the change in horizontal velocity to be calculated. The mass of the object is calculated from the horizontal movement of the ramp and the change in horizontal velocity of the object. In order to obtain accurate sensing of the movement of the ramp, the transducer is a linear variable differential transformer having a primary winding energized by a cyclical carrier signal and two matched secondary windings in series; the secondary winding output signal is rectified to a corresponding DC voltage having ripple components, and a sample is taken of the signal at the zero voltage of the principal ripple component.

BACKGROUND OF THE INVENTION

The present invention relates to apparatus for, and a method of,weighing a number of objects in succession. particularly in rapidsuccession. The intention is to provide weighing operations which do notrequire high accuracy, such as sorting objects into weight bands.

THE INVENTION

In the apparatus of the invention, there is a reaction member, means fordirecting each object in succession onto the reaction member, and meansfor giving a signal which varies with the dynamic reaction of thereaction member to the impact thereon of the object. In the method ofthe invention, each object of a number of objects in succession isdirected in turn to a reaction member, giving a signal which varies withthe dynamic reaction of the reaction member to the impact thereon of theobject, and thereby deriving a signal representative of the mass of theobject.

The term "weighing" is used although it is the mass that isdeterminative. The technique is to cause the object to interact with thereaction member, a known body, and to deduce the mass (weight) from thechange in motion of the object and change in motion of the reactionmember or force thereon, by applying the principle that the force on theobject equals the product of its mass and acceleration. The technique isa dynamic technique and not a static technique.

The invention is particularly useful for weighing objects in rapidsuccession; provided the objects are sufficiently separated so that theypass through the apparatus one after the other, they can be as close asdesired. The apparatus of the invention can be relatively cheap andsimple.

The invention is particularly suitable for weighing operations which donot require high accuracy, such as sorting the objects into weightbands. It is possible to weigh objects having a range of weights, forinstance extending from a first weight to a second weight equal to thefirst weight plus 10%; thus the invention is not just applicable tocheck weighing, where the objects are expected to have a weight veryclose to a predetermined weight. The invention can be used to weighobjects having a weight of up to for instance 5 gms, but there is notheoretical upper limit; or to weigh objects having weights down to forinstance 0.2 mg, the lower limit being determined by the resolution ofthe apparatus, so again there is no theoretical lower limit if theapparatus is sufficiently sensitive. The invention could be applied topharmacy. e.g. tablet weighing, or to weighing gem stones, for instancediamonds. In gem stone terminology, 1 carat (C)=0.2 gms and 1point=0.01C. The invention is useful for weighing relativelylight-weight rough or sawn gem stones in the range of 1-60 points,though it can also be useful for weighing stones having weights up tofor instance 1 C or more if accurate weighing is not required.

Although the apparatus has means for giving a signal which varies withthe movement of or with the force applied to the reaction member andpreferably also has means for giving a signal corresponding to thechange in velocity of the object, it is not necessary that the actualvalues be determined, provided a suitable input is given for calculatingthe mass of the objects. Likewise, the actual mass of the object neednot be calculated, though it would be normal to do so--for instance, theinvention may merely give a signal indicating the route to be followedby the object, for sorting the object into one of a number of specificweight bands. In practice, the input and output velocities in a specificdirection will vary and will each be sensed--however in theory at least,the change in velocity could be a fixed value, in which case a velocitychange signal giving means could merely be a fixed value inserted e.g.when computing the weight.

The reaction member can take various forms. In the preferred form, thereaction member has a concave surface which curves smoothly through asubstantial angle, and the objects are directed on to an initial part ofthe concave surface so that each object is guided through an angle bythe concave surface.

It is most convenient to have the concave surface curve through 90°; thechange in velocity can be measured in a specific direction parallel tothe initial direction and the final velocity (in this direction) wouldbe zero or close to zero. If the object slides around the concavesurface, the final velocity will be zero in said specific direction. Ifhowever the object bounces, there may be a relatively small finalvelocity in the said direction; to take account of this, an array ofsensors can be positioned adjacent the far end of the concave surface,for determining the resultant velocity (speed and direction) of theobject as it leaves the surface, or more simply first its component ofvelocity in said specific direction. In general terms, the advantages ofthe object being weighed whilst it is still moving are that thethroughput is high and that the object is still moving when it leavesthe reaction member, so that it is automatically removed from theweighing apparatus and can pass on for further evaluation, packaging,grouping or storage. The apparatus can thus be designed so that it canbe in a path of travel of the objects, the objects being weighed withoutstopping. It is convenient to project the objects horizontally onto theconcave surface and have the concave surface curve downwards so thatthere is no possibility of an object lodging on the concave surface.

DESCRIPTION OF PREFERRED EMBODIMENT

The invention will be further described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a side view of an embodiment of the invention;

FIG. 2 is a partial top plan view of the embodiment;

FIG. 3 is a block, schematic diagram of the function of the embodiment;

FIGS. 4 to 8 are block diagrams showing a processing system for theapparatus of FIGS. 1 and 2; and

FIGS. 9 to 13 are schematic diagrams illustrating five alternativeembodiments.

FIGS. 1-8

The apparatus has a back plate 1 from which is suspended a reactionmember in the form of a ramp 2 which has a concave surface which curvessmoothly downwards through 90°. As shown, the longitudinal section ofthe concave surface is a quarter circle but it could for instance have ashape which has a sharper curvature at one end than at the other end,e.g. in the form of a quarter-elipse, a parabola, a hyperbola, or anyother suitable curved shape, or a shape comprising a combination ofcurved and straight lines, or a shape formed of one or more straightlines. The initial part of the curve is horizontal and the final partvertical. In cross-section, the curved surface is flat but it may be anysuitable channel shape. The ramp 2 is of very light weight and may beformed of any suitable low density engineering material faced with athin, hard-wearing material (e.g. of magnesium alloy faced with hardenedsteel--steel is a good material as it has a low coefficient of frictionwith diamond).

The ramp 2 is suspended by a parallel arm linkage in the form of eightparallel springs 3 of for instance piano wire, with two rigid, lightweight, interconnecting arms 4 (a ligament suspension); for smallmovements, the ramp 2 is thus constrained to move substantiallyhorizontally. An advantage of measuring in the horizontal direction isthat gravity does not come into the calculation of weight since it doesnot affect the horizontal velocity of the object. However, if it isdesired to measure in the vertical direction, the ligament suspensioncan be put at 90°, and the transducer 5 (see below) positionedaccordingly; in general terms one can measure in any direction. Intheory, the accuracy of the weighing can be improved by simultaneouslymeasuring in two directions at right angles, movement of the ramp 2being permitted for instance in the horizontal and vertical directions.However, the improvement in accuracy does not justify the increasedcomplexity if the purpose is to sort objects at a high rate into weightranges.

A displacement transducer 5 is connected between the ramp 2 and the backplate 1, and acts as means for giving a signal which varies with thedynamic reaction of the ramp 2 to the impact thereon of the successiveobjects. In a stiff system (not shown), the transducer 5 could be aforce transducer which acts as means for determining the force in thehorizontal direction imparted to the ramp 2. In a soft system (asshown), the horizontal deflection of the ramp could be sensed in anon-contacting manner, e.g. optically or as shown by a linear variabledifferential transformer. Rigidly secured to the ramp 2, there is an arm6 carrying a hollow paddle 7 immersed in an oil bath 8, therebyproviding oil damping of the movement of the ramp 2. Alternativelyelectrical damping means could be used.

Directing or feeding means are provided to direct successive objects 9accurately in a horizontal direction onto the first part of the concavesurface of the ramp 2, at a predetermined velocity. The feeding meanscan be any suitable means, and such means are known. A suitable feedingmeans is indicated at 10. The objects 9 are delivered as indicated bythe arrow V₁ in FIG. 1, and leave the feeding means 10 in a horizontaldirection, at an input velocity V₁ of for instance 2 m/sec (in theory,the velocity should be as high as possible, but in practice is limitedby risk of damage to the objects 9 or excessive wear of the ramp 2). Thespeed of feed is accurately controlled and the feeding means 10 is shownconnected to a microprocessor 18 so as to give a signal which representsthe horizontal input velocity V₁ of the object 9. However, accuratecontrol of the feed is not essential if equipment is included formeasuring the input velocity (speed and direction).

To reduce vibration, the feeding means is not mounted on the backplate 1. To compensate for any vibration (background noise) of the backplate 1, a matched conventional accelerometer 19 is mounted on the rampsupport base or back plate 1 and its output, suitably conditioned by amodelling circuit l9a, is subtracted at 20 from the signal from thetransducer 5 before being passed to the microprocessor 18 via ananalogue/digital converter 21. This enables the apparatus to functionsatisfactorily in most working environments.

Any suitable device can be provided to signal when the object 9 leavesthe feeding means 10, primarily to signal the start of a weighingcycle--the preferred device is a split photo-diode detector 22 connectedto the microprocessor 18.

Although the input velocity V₁ can be taken as horizontal, the outputvelocity V₂ will not necessarily be vertical as the object 9 may bouncearound the ramp 2. Thus an arrangement is required which will sensemagnitude and direction of the velocity V₂ and suitable devices areknown. Suitable devices are indicated at 23, 24 and are shown asdirectly connected to the microprocessor 18.

Below the devices 23, 24, there is a continuously rotating, constantspeed carousel 25 (only part is shown) having soft, loose, nitrilerubber pockets 26 and rotating about an axis 27. The carousel motor 28is controlled by the microprocessor 18 to position a pocket 26 to catchthe object 9, and the floppiness of the pocket 26 causes the object 9 tobe decelerated and drop lightly onto double swinging flaps 29 at thebottom. The carousel 25 rotates with the object 9 within the pocket 26and after a certain degree of rotation to allow the object 9 to settleon the flaps 29, the pocket 26 passes over an array of weight-gradedreceiving bins (not shown); here a signal from the microprocessor 18causes a respective solenoid cam mechanism 30 to open the flaps 29 anddrop the object 9 into the appropriate bin.

OPERATION

The apparatus described above can be used for weighing objects in anominal weight range of 0.2 mg to 0.2 g. In operation, the horizontalvibration of the ramp 2 is analysed to determine the mass M of eachsuccessive object 9, according to the equation: ##EQU1## where x is theinstantaneous horizontal deflection of the ramp 2;

k is a calibration constant (dependent on e.g. the mass of the ramp 2,its undamped natural frequency (or stiffness of suspension) and thedamping factor);

T_(s) is a time sufficiently long for the ramp 2 to have come to restafter weighing (but before the next weighing).

ELECTRONIC PROCESSING

FIGS. 4 to 8 show a complete purpose designed weighhead printed circuitboard on which the analysis referred to above is carried out. It is asix layer board containing about 200 integrated circuits, including amicroprocessor and a `bitslice` device. It is used to carry out videoprocessing and interface with the sensors 23, 24. Functions undertakeninclude `boundary tracking`, determination of centroid of the object 9and carrying out calculation for weight. The `bitslice` device isdesigned to handle one instruction every 250 nano-secs.

The weigh-head system consists of five general elements:

(i) Linear variable differential transformer interface and rampsimulator (FIG. 4);

(ii) Interface for linear charge coupled optical sensor and video memory(FIG. 5);

(iii) Bitslice processor (FIG. 6);

(iv) Boundary tracker (FIG. 7);

(v) 16 bit processor and associated peripherals (FIG. 8).

FIG. 4 (linear variable differential transformer interface 51 and rampsimulator l9a)

The transducer 5 is a linear variable differential transformer (LVDT),which gives an alternating output voltage proportional to displacementand is used to determine the movement of the ramp 2. The LVDT has asingle primary 52 and two matched secondary windings. Movement of thecore of the LVDT, which is attached to the ramp 2, causes changes in thevoltages induced in each secondary winding. The LVDT primary isenergised by a sinewave carrier signal and the resultant sum ofanti-phase secondary signals corresponds to the displacement of the ram2. The phase of the secondary signals determines the direction of thisdisplacement. The displacement signals from the LVDT are amplified at 53and applied to a phase sensitive rectifier (PSR) 54. The PSR 54 convertsthe alternating LVDT signals into a corresponding DC voltage togetherwith ripple components at the even harmonics of the LVDT primaryfrequency. A filter 55 reduces these ripple voltages.

The interface utilises a stable reference frequency from an externalmaster clock oscillator 56 and a phase locked loop (PLL) technique togenerate the primary sinewave at a stable frequency and to synchronisethe digitisation process. The PLL consists of a phase detector 61, lowpass filter 60, a voltage controlled sinewave oscillator 59, and a highhysteresis amplifier 62. The phase of the PSR synchronisation signalrelative to the carrier can be adjusted by phase network 58 andamplifier 57. This phase lock technique reduces the effects of the highfrequency ripple which appear at the output of the PSR 54 and givesconsistently improved accuracy.

Each cycle of the LVDT primary voltage, a sample of the rectified,filtered LVDT secondary signal is taken by the sample and hold unit 63.The sample and hold control signal 64 is derived from the master clockand is synchronised to the primary voltage of the LVDT by the PLL tocoincide with the zero voltage of the principal ripple component andthus give a reduced noise component. The sample is then held constant atthe output of 63 whilst a successive approximation analogue to digitalconverter 65 generates the corresponding digital representation of thesignal (the converter has a "start conversion" input 66 and a "busy"output 67).

The resultant digital signal is read into the main microprocessor 18each period of the primary winding signal, using interrupts. Themicroprocessor 18 determines the integral of the readings over a fixedtime period 0-Ts, in accordance with the formula above. The integrationof the ramp displacement must cover the whole period the object 9 is incontact with the ramp 2 and any ramp settling time. An optical sensor(quadrant detector) 22 detects the entry of the object 9 to the rampsystem and initialises the integration process. The sensor interface isshown at 68, the input 69 being "acknowledge object fed" and the output70 being "object feed".

The ramp displacement is influenced by the impact of the object 9 andalso the vibrational motion of the machine. The latter effect must beminimised to guarantee high accuracy of estimation of object weight.This is accomplished using a destructive interference technique.

The accelerometer 19 detects the machine's vibrational motion. Theaccelerometer signal is processed through an analogue model 19a of theramp dynamics (which includes a variable gain element 72 and anelectrical model 73 of the ramp dynamics having a variable dampingcoefficient and a variable natural frequency) to give a signalcorresponding to the anticipated motion of the ramp 2. The ramp modeloutput signal is added at 74 (20 in FIG. 1) anti-phase to the actualrectified ramp signal to substantially reduce the effect of mechanicalnoise.

FIG. 5 (charge coupled optical sensor interface)

The exit trajectory of the object 9 from the ramp 2 is viewed by twolinear optical sensors 23, 24. These sensors 23, 24 are charge coupleddevices (CCD) where incident light generates charge on a number oflinearly displaced photosites. The charges (analogue) on all photositescan be inspected by transferring them to another register. This registerallows the charges to be serially shifted out and converted to the videophoto-voltages corresponding to a single line scan of the device.

The CCD's are dynamic and must be continuously driven. The drivecircuitry 81 contains a master clock oscillator and a simple sequentialtiming system. This system also generates the reference frequency andsample/hold commands for the above LVDT interface 51.

The video outputs from each CCD consist of three distinct sections:

(a) SYNC period during which the accumulated photocharge is transferredfrom photosites to the transfer register.

(b) REFERENCE period during which the output video signal shows thedark, or unilluminated level.

(c) ACTIVE VIDEO period during which the CCD shows a signal relating tothe incident light in the field of view. The CCD is normally illuminatedand thus the projectile is seen as a dark silhouette.

The video CCD signals are clamped to a fixed `black` level reference(82) during the REFERENCE period, driven by a clamping drive from theCCD drive circuitry 81. This removes any drift effects and productiontolerances from the following processing stages. The clamped signal isamplified at 83 and applied to a fast comparator 84. The peak `white`level of the signal is sampled during the ACTIVE VIDEO period andapplied to the other input of the comparator via an adjustableattenuator (white level reference) 85. This allows the white to blackthreshold of the video system to be adjusted for optimal performance.The comparator output is digitised and forced to a white level (i.e. noprojectile in field of view) during the non active video periods, by ablanking pulse.

Each video scan of the CCD 23, 24 produces 256 digital levels relatingto the 256 photosites in the respective CCD 23, 24. This digitised videois applied to a memory (256×256 bits) 86 which stores the signals for256 scans of the respective CCD 23, 24. The video scan is only storedwhen a video line contains an active pixel (i.e. projectile in view) andcontinues to store the next sequential 256 lines. Each CCD pixel memory86 thus stores a data map representing the projected silhouette of theobject 9 as it passes the respective CCD.

The position of the minimum and maximum active pixel in a row is alsogenerated using a row byte memory (256 bytes) 87. This memory 87 iszeroed prior to the arrival of the object 9. On each line scan, anyactive pixel sets a corresponding byte of the row memory 87. Thus, atthe completion of the 256 line scan, the row memory 87 indicates theminimum and maximum active pixel of the row.

The position of the maximum active line scan is generated in the sameway using a column memory (256 bytes) 88. The line scan does not proceedunless an active pixel is seen in the first line and thus the minimumactive column position is always zero (i.e. first line).

The position of the row and column minimum and maximum active pixels areused to bound the active area (i.e. area of interest) of the videomemory and so maximise speed in the image analysis and checkingprocedures.

Each two-dimensional pixel memory 86 is addresed by 8 bit row and columncounters 89 which also address the row and column byte memories 87, 88with the incorporation of multiplexers 90 controlled by the bitslicescanning controls 91. The counters 89 are shown with a counter controlbus 92, and are either controlled by the CCD drive circuit duringscanning, or by the bitslice processor (see below) during imageanalysis.

Each individual CCD interface has its own signal processing, videomemory and addressing counters 89. This allows the CCD 23 pixel memoryto be analysed by the bitslice whilst the CCD 24 memory is stillacquiring data, to minimise total image processing time. Status lines(video status processing 93, video status 94) inform the bitslice andmicroprocessor 18 of the state of the scanning (e.g. any data seen byCCD 23 or CCD 24, scan complete etc.).

The "pixel data" output 95 and bitslice buses 96 are shown.

FIG. 6 (bitslice processor system)

The bitslice processor is a customised `fast` processor designed toallow rapid image checking, analysis and centroid calculation of thedata in the CCD pixel memories 86-88. The bitslice program is held in afast 1K×88 wide read only memory (ROM) 101 associated with an 88 bitlatch 102, and consists of up to 1024 instructions each consisting of 88bits. Each 88 bit instruction can be considered in groups where eachgroup controls a particular aspect of the system:

(a) Program flow--AM2910 sequencer 103. The sequencer 103 controls theprogram flow, non-sequential program steps via conditional calls to andreturns from subroutines and conditional jumps. Immediate data can beprovided for the sequencer call and jump address, or to the bitslicedata bus 96 via buffer 104.

(b) Arithmetic logic unit control (ALU) (2 off AM2903) 106. The ALU 106allows simple arithmetic operations to be completed including ADD(with/out carry), SUBTRACT (with/out carry), and contains 16 read/writememories (RAM) and a shift unit. The control lines select the ALUfunction, RAM address and shift control codes.

(c) Shift controller, status register and condition code selector(AM2904) 107. The shift controller 107 selects the most and leastsignificant bits (MSB, LSB) for the ALU shift functions. The ALU 106 canshift both the output register and shift register and the selectorallows various combinations of LSB and MSB setups. The status registerallows the carry, zero, overflow and negative flags from the ALU 106 tobe stored in one of two registers. The stored or direct flags can besubsequently used in conditional code program control by selection.

(d) Hardware status selector (2 off AM2922) 108. This device 108 allowsselection of the various hardware flags for use in condition programcontrol. The hardware flags include scan complete, data seen by CCD,data ready from microprocessor data buffer empty to microprocessor, etc.

(e) Counter control bus 92. These signals control the many counters inthe bitslice system. Each CCD system has an X and Y counter. Eachcounter is controlled by an UP, ENABLE, and RESET signal.

(f) Scanning control 109 and tracker control 110. This group of signalscontrols the scanning to allow either or both CCD's 23,24 to be activelyscanning at any time. The tracker can boundary track either CCD 23 orCCD 24 pixel memory and the control lines facilitate this selection.

(g) A "miscellaneous" output 111 is shown.

The bitslice system closely controls the CCD scanning, data acquisition,image checking, boundary checking and centroid calculation.

Following image acquisition, the bitslice initially checks the size ofthe minimum and maxmum active row address and the maximum columnaddress. If any of these are erroneous, an appropriate error code issent to the microprocessor 18 and no further analysis is completed.Possible errors include min/max too close to address extremes (i.e. partof image may be lost), maximum-minimum row address too large (i.e.object size excessive), maximum column address too large (i.e. objectsize excessive), etc.

Following these checks, the bitslice initialises the counters to columnzero (line zero) and the row counter to the first active pixel on thisline. The boundary tracker (FIG. 7) is then run to track the boundary ofthe projected silhouette. On completion, the new boundary issequentially scanned and the centroid of the projected silhouettecalculated. The resultant data is communicated to the microprocessor 18.

FIG. 7 (boundary tracker)

The pixel memory acquires the 2-D silhouette of the object 9. In thecase of transparent or semi-transparent objects 9, this may havebreak-through (i.e. light passes through an inner region of the object9, which can happen when the object 9 has parallel sides) and this wouldgenerally reduce the accuracy of centroid calculation. The boundarytracker removes this error by tracking around the outer boundary andgenerating a new silhouette with no break-through (note that in the rarecase of edge break-through, this will not be detected and an error willoccur). The new silhouette is drawn in the upper half of the pixelmemory (column address greater than 128). The bitslice boundary checkingguarantees this half memory contains no active data prior to theboundary track.

The action of the tracker is as follows. The bitslice selects the pixelmemory to track and intialises the row counter to the first active pixelof line zero in the pixel memory. The tracker is then given control ofthe row and column counters. The complete cycle of the tracker is 8clock periods controlled by an 8 state sequence.

The states are detailed below:

(1) Save the LSB of the row and column counters 89, via a selector 121controlled by a tracker control 122, and a latch 123.

(2) Fetch data from adjacent pixel group determined by row and columnLSB saved above, via a selector 124 controlled by a tracker control 125.Each group contains 4 pixels. Save data in data combiner 126, controlledby a sequencer 127 controller in turn by a tracker control 128.

(3) Repeat step 2 for next clockwise group and merge data to form totalpixel information around our current pixel position.

(4) Repeat step 3 for next clockwise group.

(5) Repeat step 3 for next clockwise group.

(6) The data combiner 126 now contains the pixel content of all thepixels surrounding our current position. Save this data in the latch129.

(7) Using the old direction (i.e. direction entered the current pixel)and the surrounding pixel data, generate the new direction (directionfinder logic 130) to move to the next pixel. This is determined using aread only memory to cover all binary options. The direction algorithmguarantees the maximum boundary.

Latch the new direction 131.

(8) Move in the new direction to the new pixel. Signal to the bitslicethat a cycle has been completed. Allow the bitslice to read the new rowand column counters and check if the boundary has been completelytraversed. Set the corresponding pixel in the upper half of the pixelmemory (i.e. column address greater than 128). Repeat the cycle.

Once the whole boundary has been traversed, the bitslice regains controlof the counters. The boundary drawn in the upper half of the pixelmemory is now used to determine the centroid in both the row and columndirections. The centroid is determined by selecting an arbitrary centreline, weighting each pixel according to its distance from the centreline (i.e. determining its moment about the centre line), summing themoments and dividing by the number of pixels.

Whilst calculating the centroids, the bitslice also looks for any pixelsoutside the boundary and indicates these as a double or multiple feed.

FIG. 7 also shows a state decoder 132 with a "tracker status" output133, and selectors 134 and 135, the latter controlled by a trackercontrol 136.

FIG. 8 (16-bit microprocessor 18)

This is a general microprocessor which co-ordinates the whole weighingprocess. The central, 80186-type 16 bit processing unit (CPU) 141 ishighly integrated and includes internal timers, direct memorycontrollers, wait state generators etc. The CPU 141 also offersmultiplication and division functions at reasonable speed. Themicroprocessor program is contained in 16 Kilobytes of ROM in unit 142and the system has 16 Kilobytes of RAM in unit 143. The CPU 141 cancommunicate and control the bitslice by a parallel interface 144. TheLVDT digital ouput and status is also read by a parallel interface 145.The final weight is communicated to the host computer via a parallelinter-processor link (IPL) 146. This link also enables calibration andother data to be read by the microprocessor. A diagnostics interface 197is shown, which can have suitable outputs or inputs such as a VDU.

ALTERNATIVES

Using a force transducer for sensing horizontal forces, one can operateaccording to the equation: ##EQU2## where P is the force on thetransducer. In this system, the ramp and the transducer are such thatdeflection under the impact of the object is negligible--i.e. the rampand transducer would have a high natural frequency.

FIG. 9

Using a displacement transducer, the change in vertical momentum ismeasured, according to the equation: ##EQU3## (noting V₁ =V_(o) sinθ≃V_(o) θ) where y is the instantaneous vertical deflection of the ramp2.

FIG. 10

Using a displacement transducer, the change in vertical momentum isintegrated according to the equation: ##EQU4##

where

T_(s) is the vertical distance of travel of the centroid of the objectin time T_(s) ;

ω_(n) is the undamped natural frequency of the ramp; and

ζ is the damping factor on the ramp.

T_(s) can be determined by a simple optical system at the exit (atdistance Y_(s) ).

FIG. 11

Similarly to FIG. 5, the change in horizontal momentum is integrated,according to the equation: ##EQU5## where X_(s) is the horizontaldistance travelled by the centroid in time T_(s).

The systems of FIGS. 1, 2 and 9 to 11 are passive because they rely onthe natural restoring force in the ramp suspension.

FIG. 12

This system is active. A restoring force coil 151 is used, the restoringforce being proportional to the current in the coil 151. This enablesthe deflection to be larger with faster settling after weighing byvarying the effective stiffness and damping of the system.

The change in horizontal momentum is integrated, according to theequation: ##EQU6## The systems of FIGS. 1, 2, 9 and 10 can also bedevised as active systems.

FIG. 13

The ramp is suspended on an air suspension such that it is free to movehorizontally, with no natural restoring forces. The current in a coil152 can be switched on to reposition at a null point, before and afterweighing.

A conservation of momentum principle is used according to the equation:

    M[X.sub.s -V.sub.1 T.sub.s ]=M.sub.R X.sub.R

where

X_(R) is the horizontal movement of the ramp in time T_(s) ;

M_(R) is the mass of the ramp.

The present invention has been described above purely by way of example,and modifications of detail can be made within the scope of theinvention.

The idea of using the matched secondary windings of the transducertransformer 5, as described in relation to FIG. 4 above, is of generalapplicability and can be used in connection with any form of weighing ormore generally in any circuit where displacement is sensed. Statedgenerally, the idea is to sense displacement by using a linear variabledifferential transformer having a core which moves in accordance withthe displacement, a primary winding energised by a carrier signal, andtwo matched secondary windings in series, the output signal from thesecondary windings corresponding to the displacement.

The idea of storing at the wall point of the ripple, as described inrelation to FIG. 4 above, is of general applicability and can be used inany suitable circuit where a signal is being detected. Stated generally,the idea is to detect signal upon which a principal ripple component isimposed by taking a sample of the signal at the zero or mean voltage ofthe principal ripple component.

The idea of tracking around the boundary of the object to avoid errordue to break-through, as described in relation to FIG. 7 above, is ofgeneral applicability. Stated generally, the idea is to determine aparameter of an object by a method which includes tracking around theboundary of an image of the object (thereby determining the profile ofthe object)--and using information derived from the boundary whileignoring information derived from parts of the image within the boundaryin order to determine the parameter. This can be combined with the ideaof passing the object roughly at right angles to a linear sensingdevice, in order to sense the image.

We claim:
 1. Apparatus for weighing a number of discrete objects inspaced succession, comprising:a reaction member having anobject-contacting surface; means for directing each object in successionalong a substantially predetermined engagement trajectory intersectingsaid object-contacting surface, whereby the object strikes saidobject-contacting surface and the line of travel of the object ischanged through an angle substantially greater than 0° and substantiallyless than 180° by engagement of the object with the reaction member; andmeans for giving a signal which varies with the dynamic reaction of thereaction member to the impact thereon of the object.
 2. The apparatus ofclaim 1, wherein said object-contacting surface of the reaction memberis a concave surface which curves smoothly through a substantial angle,and the directing means is for directing each object in turn onto aninitial part of the concave surface so that the object is guided throughan angle by the concave surface.
 3. The apparatus of claim 2, whereinthe concave surface curves through 90°.
 4. The apparatus of claim 2,wherein the concave surface is an arc of a circle.
 5. The apparatus ofclaim 2, wherein the concave surface finishes vertically downwards. 6.The apparatus of claim 1, wherein said signal varies with the reactionof the reaction member in the horizontal direction.
 7. The apparatus ofclaim 1, wherein the object leaves the reaction member during weighing.8. The apparatus of claim 1, wherein said directing means is fordirecting the object substantially horizontally onto the reactionmember.
 9. The apparatus of claim 1, wherein said signal giving meansgives a signal which varies with the dynamic reaction of the reactionmember in a direction substantially parallel to said engagementtrajectory.
 10. The apparatus of claim 1, wherein said signal givingmeans gives a signal which varies with the dynamic reaction of thereaction member in a direction substantially at right angles to saidengagement trajectory.
 11. The apparatus of claim 1, and defining anobject departure trajectory along which each successive object leavesthe reaction member, said engagement and departure trajectories beingsubstantially at right angles to each other.
 12. The apparatus of claim11, wherein one of said engagement and departure trajectories ishorizontal and the other is vertical.
 13. The apparatus of claim 11,wherein said signal giving means gives a signal which varies with thedynamic reaction of the reaction member in a direction substantiallyparallel to one of said engagement and departure trajectories.
 14. Theapparatus of claim 11, wherein one of said engagement and departuretrajectories is substantially horizontal and the other of saidengagement and departure trajectories is substantially vertical, andsaid signal giving means is for giving a signal which varies with thedynamic reaction of the reaction member in a substantially horizontaldirection.
 15. The apparatus of claim 1, and comprising means mountingthe reaction member and permitting deflection of the reaction member onengagement by each successive object, said signal giving means giving asignal which varies with the deflection of the reaction member, and alsocomprising means for integrating the deflection of the reaction memberwith respect to time and thereby deriving a signal representative of themass of each successive object.
 16. The apparatus of claim 1, andfurther comprising electronic processor means receiving said signalwhich varies with the dynamic reaction of the reaction member, theelectronic processing means integrating with respect to time said signalwhich varies with the dynamic reaction of the reaction member andthereby giving signals representative of the weight of each successiveobject.
 17. The apparatus of claim 1, and further comprising electronicprocessor means receiving said signal which varies with the dynamicreaction of the reaction member, the electronic processing meansintegrating with respect to time said signal which varies with thedynamic reaction of the reaction member and thereby giving signalsrepresentative of the weight of each successive object, said integrationintegrating said signal from at least the instant the object engates thereaction member to the instant when the reaction member has comesubstantially to rest.
 18. Apparatus for weighing a number of objects insuccession, comprising:a reaction member; means for directing eachobject in succession onto the reaction member; means for giving a signalwhich varies with the dynamic reaction of the reaction member to theimpact thereon of the object, and comprising means for giving a signalcorresponding to the change in velocity of the object in a specificdirection during its reaction with the reaction member.
 19. Theapparatus of claim 18, wherein the object leaves the reaction memberduring weighing, and the means for giving a signal corresponding to thevelocity change comprises means for giving a signal corresponding to thevelocity in a specific direction when the object contacts the reactionmember, and means for giving a signal corresponding to the velocity ofthe object in said direction when it leaves the reaction member.
 20. Theapparatus of claim 18, wherein the object leaves the reaction memberduring weighing, the apparatus comprising an array of sensors acting asmeans for giving a signal corresponding to the velocity of the objectwhen it leaves the reaction member.
 21. The apparatus of claim 20,wherein the array of sensors comprises two linear optical sensorsextending generally normal to the path of the objects and spaced apartalong said path, and means for determining the time to pass from onesensor to the other and for determining the change in position at rightangles to said path from one sensor to the other.
 22. The apparatus ofclaim 21, and comprising means for tracking around the boundary of theimage of the object as it passes each sensor and on the basis of suchtracking determining the centroid of the object in the direction of thelinear sensor and in the direction at right angles thereto.
 23. Theapparatus of claim 18 wherein the means for giving a signal which varieswith the reaction of the reaction member comprises a displacementtransducer associated with the reaction member and in the form of alinear variable differential transformer having a core associated withsaid reaction member a primary winding and two matched secondarywindings in series, means being provided for energizing the primarywinding whereby an output signal from the secondary windings correspondsto the displacement of the reaction member.
 24. The apparatus of claim23, wherein the primary winding is energised by a cyclical carriersignal, the secondary winding output signal is rectified to acorresponding D.C. voltage having ripple components resulting from theprimary winding carrier signal, and a sampling means takes a sample ofthe secondary windings output signal each cycle of the primary windingcarrier signal, which sample is synchronished to the primary windingcarrier signal to coincide with the zero voltage of the principal saidripple component.
 25. A method of weighing a number of discrete objectsin spaced succession, comprising directing each object in turn onto areaction member whereby the line of travel of the object issubstantially changed by engagement of the object with the reactionmember, giving a signal which varies with the dynamic reaction of thereaction member to the impact thereon of the object, and therebyderiving a signal representative of the mass of the object.
 26. Themethod of claim 25, wherein the objects are gem stones.